Semiconductor lasers used as sources for an optical communications system should have the following characteristics: (1) low laser oscillation initiation current (threshold) which decreases the input power and increases the device lifetime and (2) single transverse mode operation for efficient coupling of light output to an optical fiber. The development of the DH laser structure wherein a binary active region is sandwiched between wider bandgap ternary cladding layers brought about a decrease in threshold current and made cw operation at room temperature possible. The success of the DH structure is due to the confinement of light and carriers perpendicular to the junction plane by the two heterojunctions formed at the interfaces between the binary and ternary. Lateral control of light and carriers could further reduce threshold current. Furthermore, limiting the beam width is conducive to oscillation in the lower order modes.
Stripe geometry lasers utilizing an oxide stripe or proton bombarded stripe were developed to constrain the pumping current to a narrow channel. However, the effectiveness of the stripe geometry lasers is limited by their inability to confine light parallel to the junction plane. Mesa-stripe lasers were developed wherein the active region is located in a mesa encapsulated, typically, by glass or high aluminum content ternary so that there is a large discontinuity in index of refraction which confines light to the active region. However, glass having a large discontinuity in index of refraction tends to excite higher order modes.
Buried double heterostructures wherein a filamentary binary active region is completely surrounded by a ternary have low thresholds and operate in the zero order mode. However, some means must still be employed to confine pumping current to the filamentary active region. Tsukada in Journal of Applied Physics, Vol. 45, No. 11, November 1974 suggests using the oxide stripe and selective diffusion of zinc technique to align the electrical contact with the active region.